This invention concerns a novel process for producing ceramic bodies and the ceramic bodies produced by the process. This invention also concerns a novel process for converting certain inorganic starting materials into ceramic bodies suitable for use in superconductive applications.
Some methods known for preparing ceramic bodies involve (a) hot pressing of powdered or compacted samples in a close-fitting, rigid mold or (b) hot isostatic pressing ("hipping") a sealed, deformable container having disposed therein a powdered or compacted sample utilizing a gas as the pressure-transmitting medium. In both of these methods, the sample, whether originally a powder or compact, assumes the shape of the mold or deformed container. Several problems are encountered when such methods are used. For example, the sizes and shapes of articles that can be produced are limited. In addition, finished ceramic bodies having complex shapes often contain undesirable density gradients because of nonuniform pressure distribution during pressing. Also, each sample must be compressed in a separate mold or container and, after hot pressing, the sample often adheres to the mold or container during separation.
Isostatic pressing of self-sustaining compacts (e.g., ceramic preforms) has been suggested as a possible method of overcoming the above-mentioned problems. For example, U.S. Pat. No. 3,279,917 discloses the use of a particulate material such as powdered glass or graphite as a pressure-transmitting medium in the hot pressing of refractory bodies. In this method, the particulate pressure-transmitting medium does not conform completely to the sample. As a consequence, pressure is still not transmitted uniformly and truly isostatically. Various shapes such as cubes, round rods and the like are distorted when nonuniform pressure is applied. It is therefore quite difficult, if not virtually impossible, to form intricate contours by this method.
U.S. Pat. No. 3,455,682 discloses an improved method of hot isostatically pressing (i.e., "hipping") refractory bodies which comprises the following steps: (A) surrounding the body with a mixture consisting essentially of from 5 percent to 40 percent by weight of a first component selected from alkali and alkaline earth metal chlorides, fluorides and silicates and mixtures thereof and from 60 to 95 percent by weight o a second component selected from silica, alumina, zirconia, magnesia, calcium oxide, spinels, mullite, anhydrous aluminosilicates and mixtures thereof: (B) heating said mixture to a temperature at which it is plastic: and (C) while maintaining said temperature, applying to said mixture sufficient pressure to increase the density of said body. It is taught that in this manner, low porosity, refractory bodies having a variety of shapes and sizes can be compressed to extremely low porosity and very high density without substantially altering their original shape.
U.S. Pat. No. 4,428,906 discloses a method for densifying material of metallic and nonmetallic compositions and combinations thereof to form a densified compact of a predetermined density. A quantity of such material which is less dense than the predetermined density, is encapsulated in a pressure-transmitting medium. External pressure is applied to the entire exterior of the medium to cause the predetermined densification of the encapsulated material. Densification occurs by way of hydrostatic pressure applied by the medium in response to the medium being substantially fully dense and incompressible but capable of fluidic flow, at least just prior to the predetermined densification. The invention is characterized by utilizing a pressure-transmitting medium of a rigid interconnected skeletal structure, which is collapsible in response to a predetermined force, and fluidizing means capable of fluidity. The fluidizing means is supported by and retained within the skeletal structure. The fluidizing means and the skeletal structure form a composite of skeletal structure fragments dispersed in the fluidizing means in response to collapse of the skeletal structure at the predetermined force The predetermined force renders the composite substantially fully dense and incompressible and fluidic at the predetermined densification of the compact.
U.S. Pat. No. 4,562,090 discloses a method of densifying a coating on a substrate by use of thermal and compressive stresses using a gas or solid pressure transfer medium. The substrates are steels, metal alloys or metals or non-ferrous metal alloys. The coatings are metal alloys, metals, particularly high wear and corrosion resistant metal alloys.
Surprisingly, the present invention provides a process for densifying ceramic materials by use of a combination of heat and isostatic pressure which results in a densified ceramic body, which can be in any highly complex shape at a density at a least 85 percent of theoretical density.